1. Field of the Invention
The present invention relates to a semiconductor device, a display device, a liquid crystal display device, a method for driving these devices, and a method for manufacturing these devices. The present invention particularly relates to a semiconductor device, a display device, and a liquid crystal display device which include a driver circuit formed over the same substrate as a pixel portion, and a method for driving these devices. Further, the present invention relates to an electronic device including the semiconductor device, the display device, or the liquid crystal display device.
2. Description of the Related Art
In recent years, with the increase of large display devices such as liquid crystal televisions, display devices have been actively developed. In particular, a technique for forming a driver circuit such as a gate driver over the same substrate as a pixel portion by using transistors including a non-single-crystal semiconductor has been actively developed because the technique greatly contributes to reduction in cost and improvement in reliability.
In a transistor including a non-single-crystal semiconductor, degradation such as increase in threshold voltage or reduction in mobility occurs. As such degradation of the transistor progresses, it becomes difficult to operate a driver circuit and incapable of displaying images. Patent Documents 1 and 2, and Non-patent Document 1 each disclose a shift register in which degradation of transistors can be suppressed. In these documents, two transistors are used for suppressing degradation of characteristics of transistors. The two transistors are connected between an output terminal of a flip flop and a wiring to which VSS (also referred to as negative power supply) is supplied. Moreover, one transistor and the other transistor are alternately turned on. Accordingly, the time during which the transistor is on is reduced, so that degradation of characteristics of the transistors can be suppressed.